1. Field of the Invention
The present invention relates to an optical fiber and an optical-fiber transmission line for a large-capacity optical transmission.
2. Description of the Related Art
When wavelength division multiplexing (WDM) is executed to realize large-capacity communication, a noise may be generated in an optical-fiber transmission line, which is caused by a four-wave mixing (FWM) that is a kind of nonlinear optical phenomena. To suppress this noise, for example, a nonzero dispersion-shifted optical fiber that has small wavelength dispersion at the wavelength of a signal light used for the optical transmission is used as the optical-fiber transmission line. The nonzero dispersion-shifted optical fiber has, for example, the wavelength dispersion of about 5 ps/nm/km at a wavelength of 1550 nanometers that is the wavelength of the signal light.
On the other hand, important optical characteristics of an optical fiber used in an optical-fiber transmission line in a WDM transmission system can be the effective core area and the dispersion slope characteristics. In the WDM transmission, the optical power of the signal light transmitted in the optical-fiber transmission line is increased. Therefore, to suppress the generation of the nonlinear optical phenomena caused by this increase of the optical power, it is important to lower the intensity density of the light beam in the core of the optical fiber by expanding the effective core area of the optical fiber.
In a broadband WDM transmission system aiming at large-capacity communication, to reduce the deviation of the wavelength dispersion in a wavelength band used, a dispersion flat property in the broadband, that is, a low dispersion slope characteristic is required for the optical-fiber transmission line. For example, when the S-band (1460 nanometers to 1530 nanometers), the C-band (1530 nanometers to 1565 nanometers), and the L-band (1565 nanometers to 1625 nanometers) are used as the wavelength bands, it is said to be preferable that the wavelength dispersion in each of the bands should be kept to a value between 2 ps/nm/km with which the generation of the FWM can sufficiently be suppressed and 8 ps/nm/km with which distortion of the waveform of the optical signal caused by the wavelength dispersion can be suppressed.
For an optical fiber, a trading-off relation is always present between the effective core area and the dispersion slope characteristics. Therefore, a problem is present that it is difficult to simultaneously realize expansion of the effective core area and a low dispersion slope characteristic.
FIG. 37 is a table of optical characteristics at the wavelength of 1550 nanometers of nonzero dispersion-shifted optical fibers respectively of a large-Aeff-type having the expanded effective core area, a low-slope-type having a reduced dispersion slope, and an ultralow-slope-type. In FIG. 37, “Slope” represents a dispersion slope and “Aeff” represents an effective core area. FIG. 38 is a graph of the relationship between the wavelength and the wavelength dispersion of each nonzero dispersion-shifted optical fiber shown in FIG. 37.
As shown in FIG. 37: the large-Aeff-type nonzero dispersion-shifted optical fiber has the effective core area that is expanded to 70 μm2 or larger; and the low-slope-type and the ultralow-slope-type nonzero dispersion-shifted optical fibers have the dispersion slope that are reduced to 0.045 ps/nm2/km and 0.02 ps/nm2/km, respectively (see e.g., Y. Liu, et al., “Single-mode dispersion-shifted fibers with large effective area for amplified systems”, IOOC 1995, PD2-9 (1995), D. W. Peckam, et al., “Reduced dispersion slope, nonzero dispersion fiber”, ECOC 1998, p. 139 (1998), and N. Kumano, et al., “Novel NZ-DSF with ultralow dispersion slope lower than 0.020 ps/nm2/km”, ECOC 2001, PD. A. 1. 5 (2001)).
On the other hand, a technique is disclosed that uses a multi-mode optical fiber as a technique of significantly expanding the effective core area (see, e.g., Japanese Patent Application Laid-Open No. 2004-2719049).
However, problems have been arisen that the above large-Aeff-type nonzero dispersion-shift optical fiber has the dispersion slope of about 0.09 ps/nm2/km that is large and, therefore, the deviation of the wavelength dispersion for the entire optical-fiber transmission line can not be sufficiently reduced, and that the low-slope-type nonzero dispersion-shift optical fiber has the effective core area of about 50 μm2 that is small and, therefore, the generation of the nonlinear optical phenomena can not sufficiently be suppressed.